One method for utilizing coals has been recently proposed. That is, the coals are pulverized and mixed with a small amount of water for slurrying with a high density and pasting in order to enable the transportation using a pipeline or the like. The product obtained by such a method is referred to as the high-density coal-water mixture or slurry (this will be abbreviated as CWM hereinafter) or the high-density coal-water paste (this will be also abbreviated as CWP hereinafter).
In case of the CWM, the density of the coal is increased to 65 through 70 wt % by adjusting the coal particle size distribution to provide the fluidity, and the coal is directly burned in an ordinary boiler without dehydration. Meanwhile, in case of the CWP, the coal particle size distribution is adjusted so that the particle size can be equal to or less than 6 mm which is slightly larger than that in the CWM, and water is added to the coals together with the desulfurizing agent to provide the density of 70 through 80 wt % to give the fluidity. The CWP is then pushed out from the pipeline into a fluidized bed combustion boiler by a pump and burned without making any change. In order to carry out these processes, the water density is decreased as low as 30 through 35 wt % and the sufficient fluidity is required in the CWM or CWP.
Although the method for producing the CWM or CWP has been already commercialized in the wet manufacturing method utilizing the wet grinding, the stronger grinding power is required when carrying out the wet grinding, which increases the manufacturing cost. Development of the dry manufacturing method utilizing the dry grinding with the reduced grinding power is thus desired. In the dry manufacturing method, drying the pulverized coals during pulverization provides the strong water repellency and makes the slurrying difficult. Therefore, in the conventional CWM or CWP production, in order to facilitate flow using the pipeline by slurrying the pulverized coals having the strong water repellency, it is required to add 0.1 through 1 wt % of the dispersant having the general surface active agent as a main component which may be substituted any other material depending on properties of the surface active agent when producing the CWM having a high density of, e.g., 65 through 70%. This improves the wettability of the pulverized coals and prevent aggregation of the pulverized coals in water. Of course, it is similarly necessary to add a large amount of the surface active agent in the wet producing method in order to improve the wettability of the pulverized coals and prevent aggregation of the pulverized coals.
However, in the CWM or CWP described above, the cost of the dispersant per unit is relatively high, and hence the cost of the dispersant accounts for about 20 to 40% of the cost of the CWM or CWP, even the amount of the dispersant accounts for 0.1 through 1 wt % of the amount of the CWM or CWP.
Various kinds of dispersant have been proposed for reducing the cost of the dispersant. For example, although the dispersant which has a high efficiency and whose amount can be reduced has been developed, this type of dispersant disadvantageously increases the cost per unit. Further, the inexpensive dispersant has been also developed, but an amount of this dispersant to be added must be increased. Thus, reduction in the cost of the dispersant is difficult, and hence the cost of the CWM or CWP can not be lowered.
In addition, the fluidity of the CWM or CWP depends on how the particles fill. The middle-sized particles enter into a gap between the large particles, and the small particles enter into a gap between the middle-sized particles. Further, the superfine particles enter a gap between the small particles, and water enters a gap between the superfine particles. This small amount of water generates the fluidity, and the superfine particles which exist around the relatively large particles having the size not less than a few .mu.m and have the size of approximately 1 .mu.m serve as the lubricant, thus assuring the fluidity.
However, in the conventional CWM or CWP producing method in the dry manufacturing method, the pulverized coal obtained by the dry pulverization has an unspecified substantially-polyhedral angular shape, and a large gap is then made between the particles. The gap is not filled even though the regularly-generated amount of superfine particles is introduced, thereby making realization of the high density of the CWM difficult. Furthermore, even if realization of the high density of the CWM is possible, the lack of the superfine particles causes the relatively-large coal particles (a few .mu.m or more) to come into contact with each other without the superfine particles, thereby making the enhancement of the fluidity difficult.
In order to realize the high density and enhance the fluidity of the CWM or CWP, there is considered a method such as that a large amount of coal particles having the size of approximately 1 .mu.m which are referred to as superfine particles is prepared and mixed and such particles are provided between the large coal particles.
However, in the above-described CWM or CWP producing method, since a great amount of superfine particles which are relatively difficult to be pulverized is required, and the mass production is hard to be effected, which actually leads to difficulty in reduction in the manufacturing cost. It is to be noted that the CWM and CWP are generally referred to as the high-density coal-water mixed fuel in this specification and the high-density coal-water mixed fuel includes the high-density coal-water paste as well as the high-density coal-water slurry unless it is specified.